ISO / Sensitivity accuracy

In a new addition to our reviews we are now measuring the actual sensitivity of each indicated ISO sensitivity. This is achieved using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV.

In real terms the DSLR-A100 matches the Canon cameras in being about a third more sensitive than indicated (ISO 100 is more like ISO 125).

Indicated
sensitivity

Sony DSLR-A100 (actual sensitivity)

Canon EOS 30D(actual sensitivity)

ISO 100

ISO 125

ISO 125

ISO 200

ISO 250

ISO 250

ISO 400

ISO 500

ISO 500

ISO 800

ISO 1000

ISO 1000

ISO 1600

ISO 2000

ISO 2000

ISO 3200

n/a

ISO 4000

ISO Sensitivity / Noise levels

ISO equivalence on a digital camera is the ability to increase
the sensitivity of the sensor. The works by
turning up the "volume" (gain) on the sensor's signal amplifiers (remember the sensor is an analogue device). By amplifying the signal you also amplify the noise which becomes more visible at higher ISO's. Many modern cameras also employ noise reduction and / or sharpness reduction at higher sensitivities.

To measure noise levels we take a sequence of images of a GretagMacBeth ColorChecker chart (controlled artificial daylight lighting). The exposure is matched to the ISO (ie. ISO 200, 1/200 sec for consistency of exposure between cameras). The image sequence is run through our own proprietary noise measurement tool (version 1.4 in this review). Click here for more information. (Note that noise values indicated on
the graphs here can not be compared to those in other reviews). Room temperature is approximately 22°C (~72°F), simulated daylight lighting.

What to compare?

The DSLR-A100 doesn't really fit into any existing D-SLR category, its price hints at the higher-end amateur market against cameras such as the Canon EOS 30D and Olympus E-330, however that will leave you with some $300 change compared to the EOS 30D. As you can see a direct comparison isn't easy with a range of resolution and price. For the purposes of comparing noise we'll use the eight megapixel Canon EOS 30D (which also effectively has the same noise response as the EOS 350D / Digital Rebel XT) and seven-point-five megapixel Olympus E-330.

From ISO 100 to 400 there's virtually nothing between the DSLR-A100 and EOS 30D (noise levels while higher are still so low as not to be noticeable), at ISO 800 however the difference between these two cameras becomes visible and at ISO 1600 the A100 has significantly more noise. On the counter side I would say I'm pleased to see that despite the loss of some detail at ISO 1600 the A100's noise reduction algorithm appears to be tuned to maintain image detail where possible.

Luminance noise graph

Our new luminance noise graph shows noise levels in both the middle gray and black patches. As you can see the DSLR-A100's gray luminance noise plot follows the EOS 30D (albeit at a higher level) up to about ISO 800 after which it takes a noticeable jump. The biggest difference is in the black patch (shadow) noise which takes a leap on the DSLR-A100 above ISO 400.

Indicated ISO sensitivity is on the horizontal axis of this graph, standard
deviation of luminosity on the vertical axis.

Chroma (color) noise graph

In addition to measuring luminance noise we now also measure chroma (color) noise. If you look at the patches above you'll see that the most noticeable noise in the gray patches at ISO 800 and 1600 looks like colored mottle, that's chroma noise and the difference between the DSLR-A100 and E-330 versus the EOS 30D is pretty obvious in this graph. It's a pity Sony didn't employ Nikon's technique of working mostly on reducing chroma noise (as this is the most 'digital looking'), luminance noise isn't as visually objectionable.

Indicated ISO sensitivity is on the horizontal axis of this graph, standard
deviation of each of the red, green and blue channels are
on the vertical axis.